Antigen-specific T lymphocyte activation occurs through the clonally distributed T cell receptor (TCR). The available set of alpha beta TCR is dictated by both positive and negative intrathymic selection events. The molecular basis of this selection, in particular, how bias for self-MHC presented peptides is achieved, is unknown. Furthermore, the quantitative and qualitative relationships between receptor occupancy and signaling for differentiation, whether in the thymus or the periphery, are also poorly understood. This project uses cellular and molecular tools to study the development of the T cell repertoire and the activation of T cells upon ligand engagement. Analysis of thymocyte subsets in normal and MHC class I-deficient mice allowed the identification of a novel thymocyte subpopulation (CD4 intermediate CD8hi TCRhi) that is in the process of becoming a CD8+ mature T cell following receptor recognition of MHC class I ligand. These studies unexpectedly revealed that some cells initiate development in the CD8 lineage by recognition of MHC class II molecules. This finding casts doubts on a model of T cell differentiation that ascribes initial lineage commitment to a match between the binding specificity of the CD4 or CD8 co-receptor and the MHC class of TCR ligand. We have found that some TCR ligands evoke only a subset of T cell effector responses, and that such variant ligands can antagonize cytokine production induced by known receptor agonists. These findings may provide a new approach to the control of autoimmune diseases and a means of controlling the type of effector responses elicited by component vaccine formulations. Finally, because IL-2 (T cell growth factor) production by CD4+ T cells is known to be very dependent on co-stimulatory signals distinct from those arising from occupancy of the clonally distributed alpha beta T cell receptor, we have begun exploring the pathways that lead to expression of the major co-stimulatory ligand( B7) on B cells. Our work demonstrates a critical role for membrane-mediated T cell: B cell signaling involving the interaction of T cell-expressed CD40 ligand and B cell expressed CD40, and suggests that expression of CD40 ligand (the locus of the defect in the hyper IgM immunodeficiency syndrome) may require co-stimulatory signals.